Abstract: GPS radio occultation measurements aboard the GRACE-A satellite were continuously activated on May 22, 2006. The data are automatically analyzed by the GFZ orbit and occultation processing systems. About 150 daily globally distributed vertical profiles of refractivity, temperature and water vapour are derived. A substantial number of these data are provided with an average latency between measurements and availability of corresponding analysis results of less than 5 hours. Refractivity data are already operationally used by the MetOffice since end of September 2006 to improve global weather forecasts.

The history of GPS radio occultation on GRACE from its initial short activation mid 2004 is reviewed. Results from GRACE-A and -B orbit and occultation analyses are presented and profiles are validated with independent meteorological data. Instrument specifics compared to the German CHAMP satellite are characterized, and the zero differencing technique for the GRACE occultation data analysis is introduced. We also review the use of the GRACE data by various weather centers and present results of an initial assimilation study to improve global weather forecasts using CHAMP and GRACE data at ECMWF. Finally we give an outlook on the capabilities of the current multi-satellite occultation constellation of CHAMP, GRACE, COSMIC and METOP.

Abstract: The success of the GRACE mission depends on many factors, including the stability of the GRACE Ultra Stable Oscillators (USOs) used for the microwave link. Pre-launch measurements demonstrated that the oscillators had stabilities (Allan variance) of 1-3x10-13 for periods of 2-1000 seconds. Post-launch data subsequently showed short-term repeatabilities of better than 150 ps in 6 hour overlap tests. In this study, we examine the GRACE USO performance over a longer period (6 months) and assess the dominant errors in the GPS timing estimates for the USOs. These Allan variance calculations for the GRACE timing systems include both USO noise and GPS transfer-noise. Our first task was to correct the timing estimates for relativistic effects. A proper model for this effect removes eccentricity related timing errors with a dominant once/rev period. Our relativistic model also includes the effects of J2. After relativistic effects have been removed, both GRACE USOs show once/rev signatures with amplitudes of 3.4 and 2.4 ns for GRACE A and B, respectively. Such variations are consistent with temperature or voltage sensitivities reported by the manufacturer of the USOs. We found no thermistor data with large enough variations to explain the GRACE USO once/rev signature. On the other hand, once/rev voltage variations are significant (peak to peak of 5 volts) and are in phase with the GRACE USO variations. The presence of the large once/rev signature in the GRACE USOs significantly degrades oscillator performance (8-20x10-13) at periods of 300-2500 seconds.

Abstract: Accurate thermospheric density and wind estimates can be obtained from the GRACE and CHAMP accelerometers. Although not part of the main GRACE goals, these data are an important and under-utilized resource that can be used to better understand thermospheric density and wind structures, and their variability. There has previously been relatively little high quality data available on thermospheric density and winds, and there are many features that still require detailed study. We describe in general terms the drivers of thermospheric variability, and show some initial examples of GRACE and CHAMP density and wind data. The GRACE density data are compared with density estimates from the Global Ultraviolet Imager on TIMED. Simulations from a global first-principles model (TIMEGCM) provide a framework for interpretation of the data, and suggest some interesting experiments that could be performed by combining the GRACE and CHAMP datasets.

Abstract: There is significant interest in specifying ionospheric electron densities, and the GRACE mission provides three different types of ionospheric measurements. First, information on the altitude distribution of ionospheric electron density between the GRACE satellite altitude and ~ 100 km is provided by total electron content (TEC) occultation data. Second, relative TEC along the path between the satellites is provided by the inter-satellite radar range measurements. Estimates of the change in electron density along the satellite trajectory can be made from such observations. Third, integrated information of the topside ionosphere and plasmasphere are obtained from TEC measurements between the GPS satellites and the onboard NAVIGATION GPS receivers. This third measurement is particularly interesting and unique because it is made by both GRACE satellites and therefore by taking the difference of the two observations for each GPS satellite, a measurement of the change in ionospheric electron density along the path of the satellites is provided. GRACE is the only satellite capable of providing such information.

The GRACE data are particularly useful for ingestion by assimilative algorithms. Ionospheric Data Assimilation Three-Dimensional (IDA3D) is an ionospheric data assimilation algorithm currently available for scientific investigations at ASTRA. IDA3D is a 3DVAR Kalman filter algorithm that is capable of ingesting any data source that provides information on ionospheric electron density. Since occultation TEC from a number of different LEO satellites has been widely reported in the literature, the inter-satellite estimate of electron density, and the TEC measurements of topside and plasmaspheric content will be the focus of this presentation. First, comparisons between GRACE estimates of insitu electron density and IDA3D during the magnetic storm of November 20, 2003 will be presented to demonstrate the validity of the GRACE measurement. Then, the impact of ingesting GRACE topside TEC into IDA3D is demonstrated through presentation of IDA3D analysis on the November 20, 2003 and October 30, 2003 magnetic storms. Finally, gradients in ionospheric content along the GRACE trajectory during magnetic storms obtained from observations of the difference in the topside TEC between two GRACE satellites is presented.